Electrical pick-up device for phonograph records



' 1,637,165 1.. THOMPSON ET AL ELECTRICAL PICK-UP DEVICE'FOR raouosrmrn macoans Filed April 15, '1927 July 26, 1927.

m mummumm\ INVEN TORS Patented July 26, 1927.

UNITED STATES IPATENT OFFICE.

LINCOLN THOMPSON, OF NAUGATUCK, AND JOSEPH B. MARSHALL, OF WESTPORT,

CONNECTICUT, ASSIGNORS TO THE BRISTOL COMPANY, OF WATERBURY, CONNECTI- CUT, A CORPORATION OF CONNECTICUT.

ELECTRICAL PICK-UP DEVICE FOR PHONOGRAPH RECORDS.

Application filed April 13, 1927. Serial No. 183,377.

The invention relates to apparatus utilized in the reproduction of sound; and more especially to an electrical transmitter unit for use with phonographic records of the leteral cut type, the grooves of which are adapted to impart an oscillatory movement I i a stylus member operating therein.

It has for its object a sound reproducing device of the aforesaid character wherein a vibratory element as a reed of magnetic material is caused to operate substantially free from mechanical and electrical resonances in anclectromagnetic field, causing electrical currents to be generated in a transl5 mitting c011 surrounding this vibrating member or reed, whose vibration is imparted thereto by an attached stylus which engages the grooves of a phonographic record. To this end, the reed is located for vi- 2 bration within a transmitting coil and is flexibly mounted so as to vibrate from an interrupted portion of a magnetized frame having separated pole pieces disposed opposite the interrupted ortion. This vibration changes the magnetic path of the lines of force passing from one pole to the other, and causes movements of magnetic flux up and down the reed corresponding to the vibration imparted to the latter by the stylus engaging the grooves of the record, which movement of flux in the reed in turn causes electrical currents to be generated in the transmitting coil conformably to the direction of the movement of the flux and consequently corresponding to the vibratory movements of the reed. It will be understood that these currents, as produced indirectly in this manner from the phonographic record, are not of suflicient magnitude to operate the ordinary existing receiver devices with much volume; but, when suitably amplified by any of the well-known cxpedients and the amplifier output applied to a. loud speaker, the recordings of the record may be reproduced at great volume.

A further object of the invention resides in a transmitter or pick-up member of this type which shall be of substantial and ru ged construction, without undue weight an withal suitable for transferring the effect of mechanical undulations of the moves of a phonographic record with which the stylus is in engagement into corresponding electrical currents w t gr a fid l y and. in

manner such that, with well-known amplifying methods of high quality and well-known loud speaker devices of high quality, the purity of the original recording will be maintained at great volume and with a minimum of foreign sounds such as objectionable record scratch and metallic sounds.

The nature of the invention, however, will best be understood when described in connection with the accompanying drawings, in which:

Fig. 1 is a side elevation of the novel pick-up device and mounting therefor, and shows also a stylus of said device in engagement wit-h a phonographic record.

Figs. 2 and 3 are respectively a front and side elevation, on an enlarged scale, of the pick-up device with a portion of the cover plate broken away in Fig. 2 to disclose the vibratory reed and surrounding coil of wire.

Fig. 4 is an underneath view of the pickup device.

Fig. 5 is a transverse section taken on the line 55, Fig. 2, and looking in the direction of the arrows.

Referring to the drawings, the novel pickup device is arranged to be mounted in any suitable manner for floating over the top of the phonographic record- 10 of the wellknown lateral-cut, disk type and arranged to be carried by a turn-table 11 by which it may be rotated in manner well understood. Any suitable means for thus supporting the pickup device with reference to the turn-table and record thereon may be utilized; and in the drawings there is indicated/a standard or base 12 of substantial proportions, the same having a socket 13 which is designed to receive a pin 14 for rotation therein so that the ick-up device may be caused to swing free y in a horizontal plane. The latter is designed, also, to be carried for this purpose by an arm 15, preferably in the form of a tube through which the outgoing leads or conductors 16 may be brought for connecting the pick-up device with the external circuits and apparatus (not shown). The pickup end of the arm 15, moreover, is provided with a strap member 17 for rigidly securing to the arm the said pick-up member'as by means of screws 18. The opposite or inner end ofthis arm is pivotally secured to the upper end of pin 14 to provide for a. universal joint which pin, for thls purpose, term1 nates in a bifurcated portion 19 having inwardly directed studs 20 upon which the arm 15 may rock in a vertical plane as in insert ing the stylus 21 of the pick-up member in the grooves of the record 10 and removing the same therefrom.

The pick-up member comprises a magnetized frame 25, preferably of the U or horse shoe type, and also preferably permanently magnetized. The outer portion of this magnet carries inwardly directed U- shape extension pieces 26 and 27, the outer most legs of which are beveled down to afford pole pieces 28 and 29 with intermediate air gap 30; while the innermost legs of the respective U-shape pieces are separated to provide an interrupted portion in the magnetic circuit-the magnetism of the two pieces being maintained by the frame or U- shape magnet 25.

The respective sides of the U-shape pieces have secured thereto plates 31 and 32 respectively and of non-magnetic material, said plates having exterior hubs 33 and 34, respect'ively, for receiving the opposite ends of a shaft 35. The latter is designed to be fixedly held therein by means of pairs of-suitable screws 36 and 37 passing through the respective hubs to bear upon the said shaft ends.

Shaft 35, furthermore, is reduced intermediate its ends as at 38 and 39 to a degree pro vidmg the desired torsional strength for holding the attached soft iron reed member 45 in its normal position in the gap between pole pieces and equidistantly therefrom and in opposition to and against the magnetic pull exerted by these poles. The degree of this reduction also greatly alfects the quality of the pick-up device, since the vibrating system thereof depends for its proper action mainly upon the torsional stiffness of this shaft and the mass of the attached reed which is to be vibrated on this shaft as a torsional axis. The reed 45 to this end is rigidly attached at the middle part of shaft 35 and at right angles thereto so that the portion of the reed where it is attached to the shaft partially bridges the interrupted portion of the extension pieces, Fig. 5 of the drawings. The other end of the reed, that is, its free end, passes beyond the pole pieces and is held in the aforesaid manner in its normal position equidistantly therefrom.

Since the combination of the effective torsional stifi'ness of the shaft 35 and the efiec tive mass of the reed vibrating about this torsional axis will lead to a mechanical natural period which will cause the vibratory force applied to the reed required to produce a given velocity to be much lower at this point of resonance than at any other frequency, a pair of pads 46 of resilient material of relatively high mechanical resistance is provided to cause a greater uniformity of applied force for a given velocity at all frequeneies, or, in other words, to damp the natural period of the system. These pads, which may be of rubber, are secured in boxes or cap members 47 attached to the outer sides of the respective pole pieces by screws 18 and their pressure against the reed may be adjusted, by screws 49 acting against the backs of the pads and passing through the cap members, to give the required degree of damping; or, in other words, to adjust the mechanical resistance of the vibrating system.

The reed 45 terminates in a socket 50 for receiving the stylus 21 which is held by a set screw 51 and preferably hollowed out in order to decrease to a minimum the effective vibratory mass of the system.

It will be understood that as the stylus operates in the grooves of the record 10, the reed will partake of the motion of said stylus and be moved closer to one of the pole pieces and further from the other, and vice versa, as determined by the motion imparted to the stylus. The major part of the flux will flow through the shortest air gap. so the flux will flow in one direction through the reed when it is moved closer to one pole piece, and in the reverse direction when the reed moves closer to the other pole piece. These movements of flux will generate electrical currents in a coil 55 surrounding the reed. the currents varying in direction with the direction of the flux flow along the reed and so varying in direction with the direction of motion of the reed toward the one pole or the other under the influence of the stylus engaged in the groove of the record.

It will be understood that the said coil of wire 55 about the reed is rigidly held within the extension pieces 26 while the reed itself is free to move within the coil. The ends of -the latter in turn are attached to the terminals 56 connected with the leads 16 passing through arm 15 as hereinbeforc set forth; and these leads may be suitably attached to a receiver device (not shown) for musical reproductions of low volume or to the impact of a suitable amplifier (not shown) for greater volume.

It will be noted that this construction embodies many valuable features. In the first place, it is a vibrating system comprlsmga reed of such dimension as not to flex, said reed being attached to a torsional shaft whose dimensions are reduced at two points to a degree providing the desired elasticity or stiffness so that all motion is transmitted up the reedwith substantially no flexure thereof, all flexure taking place atthe torsion roducing portion of the supporting shaft. he efiective mass of the rec plus the mass of the attached needle and set screw, as it oscillates about its end supported by the torsional shaft, together with the elasticity of the torsion producing portions, results 11111 natural period of vibration of the system; but the proportioning of these factors is made such that the natural period has a high damping factor, that is, if the system be glven an impact, the resultant vibration in its natural period would be of much shorter duration than if the same natural period were obtained with a much lighter mass and greater torsional stitlness'of the shaft portions. However, the mass is not so great that too much energy would be required from the record to move it and thus cause excessive record wear. Moreover, in order to further reduce any ill effects due to this natural period, the mechanical resistance of the pads, as applied to the reed at its extreme tip, provides effective damping. Also, it will be noted that the reed is cut oil as close as possible to the gap consistent with convenience in inserting and holding the stylus, and thus the motion'imparted to the tip of the stylus need only travel a very short distance to the gap and consequently the motion of the reed in the gap follows very faithfully the motion imparted by the record to the stylus tip. It is to be observed that the socket for the stylus is relatively deep, thus also favoring a very short distance of travel between record groove and gap. It will be' further noted that the needle is at the extreme tip of the vibrating system, and, since the Weight of the whole device is sufficient to hold it firmly in the groove, the groove itself is a damping agent on the vibratingsystem; and with the very short distances from the stylus tip to its clamping point in the reed, comparatively little fiexure is possible and therefore there is practically no loss of efliciency due to fiexure. To sum up, the invention comprises a vibrating system having a natural period whose ill effects are rendered negligible by a summation of factors separately and differently militating against any harmful effects from it-first, the combination of mass and stiffness is such to cause a high damping factor; secondly, mechanical damping resistance further damps the natural fre quency; and, thirdly, the mechanical transmission is such that the record groove itself acts as a damping medium at the point of maximum leverage'of the system. It will be further noted that with this system of transmission of mechanical motion. the motion in the gap is less than the motion imparted to the stylus point, whereas some other systems have more motion in the gap than the stylus point receives. However, this decrease of motion has been compensated for by using correspondingly smaller air gaps.

We claim:

1. An electrical transmitter for phono graphs, comprising: a magnetized metal frame having an interruption at one end, and a pair of separated pole pieces at the opposite end; a transmitting coil of wire carried by the magnetized frame; a reed .member torsionally secured at the intertween the said poles when the stylus is en-- gaged by the grooves of a phonographic record.

2,. An electrical transmitter for phonographs, comprising: a magnetized metal frame having an interruption at one end, and a pair'of separated pole pieces at the opposite end; a transmitting coil of wire carried by the magnetized frame; a reed member torsionally secured at the interrupted portion and extendingv outwardly through the coil and the air gap between the pole pieces and adapted to be attracted thereby but maintained normally substantially equidistant therefrom by its torsion support, said reed terminating at its outer end in a socket for retaining a stylus whereby said reed may be vibrated as a whole between .the said poles when the stylus is engaged by the grooves of a phonographic record; and damping members carried by the magnetized frame for mechanically damping the vibrations of the reed.

S. An electrical transmitter for phonographs, comprising: a magnetized metal frame having an interruption at one end, and a pair of separated pole piecesat the opposite end; a transmitting coil of Wire carried by the magnetized frame; a reed member torsionally secured at'the interrupted portion and extending outwardly through the coil and the air gap bet-ween the pole pieces and adapted to be attracted thereby but maintained normally substantially equidistant therefrom by its torslon support, said reed terminating at its outer end in a socket for retaining a stylus whereby said reed may be vibrated as a whole between the said poles when the stylus engaged by the grooves of a phonographic record; and rubber pads carried by the pole pieces adapted to engage the reed.

4. An electrical transmitter for phonographs, comprising: a magnetized metal frame having an interruption at one end, and a pair of separated pole pieces at the opposite end; a transmitting coil of wire carried by the magnetized frame: a shaft extending through the interrupted. portion of the magnetized frame. said shaft having end reduced portions whereby it may have a limited oscillation about its longitudinal axis and exert a predetermined torsional stress; and a reed member fixedly attached to the shaft and extendingat right angles therefrom outwardly through the coil and the air gap between the pole pieces and adapted to be attracted thereby but maintained normally substantially equidistant therefrom by its torsion support, said reed terminating at its outer end in a socket for retaining a stylus whereby said reed may be vibrated as a whole between the said poles when the stylus is engaged by the grooves of a phonographic recor 5. An electrical transmitter for phonographs, comprising: a magnetized metal frame having an interruption at one end, and a pair of separated pole pieces atthe opposite end; a transmitting coil of wire carried by the magnetized frame; a shaft extending through the interrupted portion of the magnetized frame substantially fillin the interrupted portion of the magnetize frame and fixedlv secured to said frame, said shaft having end reduced portions whereby it may have a limited oscillation about its longitudinal axis and exert 'a predetermined torsional stress; and a reed member fixedly attached to the shaft and extending at right angles therefrom outwardly through the coil and the air gap between the pole ieees and adapted to be attracted thereby ut maintained normally substantially equidistant therefrom by its torsion support, said reed terminating at'its'outer end in a socket for retaining a stylus whereby said reed may be vibrated as a whole between the said poles when the stylus is engaged by the grooves of a phonographic record. I

In testimony whereof We affix our signatnres.-

LINCOLN THOMPSON. JOSEPH B. MARSHALL 

